In the 1820s George IV engaged the architect John Nash to redevelop the area. Nash cleared the square as part of his Charing Cross Improvement Scheme. The present architecture of the square is due to Sir Charles Barry and was completed in 1845.

Trafalgar Square is owned by the Queen in Right of the Crown and managed by the Greater London Authority, while Westminster City Council owns the roads around the square, including the pedestrianised area of the North Terrace.[2] It forms part of the Northbank business improvement district.[3]

The square consists of a large central area with roadways on three sides and a terrace to the north, in front of the National Gallery. The roads around the square form part of the A4 road. The square was formerly surrounded by a one-way traffic system, but works completed in 2003 reduced the width of the roads and closed the northern side to traffic.[4]

Nelson's Column is in the centre of the square, flanked by fountains designed by Sir Edwin Lutyens in 1937–39 as replacements for two earlier fountains of Peterhead granite (now in Canada) and guarded by four monumental bronze lions sculpted by Sir Edwin Landseer. The lions were cast in bronze melted down from the cannons aboard French and Spanish ships that had taken part in the battle. The column is topped by a statue of Horatio Nelson, the vice admiral who commanded the British Fleet at Trafalgar.

From 1732, the King's Mews were divided into the Great Mews and the smaller Green Mews to the north by the Crown Stables, a large block, built to the designs of William Kent. Its site is now occupied by the National Gallery. The Royal Mews were transferred to Pimlico in the 1820s, and the stable block used as a menagerie, and for the storage of public records, until its demolition in 1835.[5] In 1826 the Commissioners of H.M. Woods, Forests and Land Revenues instructed John Nash to draw up plans for clearing a large area south of Kent’s stable block, and as far east as St Martin’s Lane. His plans left open the whole area of what was to become Trafalgar Square, except for a block in the centre, which he reserved for a new building for the Royal Academy.[6] They also involved the demolition and redevelopment of an area of buildings between St Martin’s Lane and the Strand, and the construction of a road (now called Duncannon Street) across the churchyard of St Martin-in-the-Fields.[7] The Charing Cross Act was passed in 1826 and clearance of the ground started soon after.[6]

After the initial clearance, development of the square progressed slowly. The National Gallery was built on the north side in 1832–38 to a design by William Wilkins,[6] and in 1837 the Treasury approved Wilkins’ plan for the laying out of the square itself, but it was not put into effect.[8] In April 1840,[9] Wilkins having died in the meantime,[6] new plans by Charles Barry were accepted, and construction started within weeks.[9] For Barry, as for Wilkins, a major consideration was increasing the visual impact of the National Gallery, which had been widely criticised for its lack of grandeur. He dealt with the complex sloping site by excavating the main area of the square down to the level of the footway between Cockspur Street and the Strand,[10] and constructing a fifteen foot high balustraded terrace with a roadway on the north side, with steps at each end leading down to the main level.[9] Wilkins had proposed a similar solution, but with a central flight of steps.[8] Plinths were provided for sculpture and pedestals for lighting. All the stonework was of Aberdeen granite. The estimated budget, excluding paving and sculptures, was £11,000.[9] The earth removed was used to level Green Park.[10] The next year it was decided that two fountains should be included in the layout.[11] The square was originally paved with tarmacadam. This was replaced with stone in the 1920s.[12]

Nelson's Column had been planned independently of Barry’s work. In 1838 a Nelson Memorial Committee had approached the government, proposing that a monument to the victor of Trafalgar, funded by public subscription, should be erected in the square, and the government had provisionally agreed. A competition was held, the winning design, by the architect William Railton, being for a Corinthian column topped by a statue of Nelson, with an overall height of more than 200 feet, guarded by four sculpted lions. The design was approved, with the proviso that the overall height should be reduced to 170 feet, and construction began in 1840.[9] The main construction of the column was completed, and the statue raised in November 1843.[13] However, the last of bronze reliefs on the pedestal of the column was not installed until May 1854, and the four lions, although part of the original design, were only added in 1867.[14]

Barry was unhappy about Nelson’s Column being placed in the square. In July 1840, when its foundations had already been laid, he told a parliamentary select committee that "it would in my opinion be desirable that the area should be wholly free from all insulated objects of art".[9]

The hoardings were removed and the square opened to the public on 1 May 1844, although the asphalt paving was still soft, and the fountains were not playing. There was still a hoarding around the base of Nelson’s Column, which was to remain for some years, and some of its upper scaffolding remained in place.[15]

The square has become a social and political focus for visitors and Londoners alike, developing over its history from "an esplanade peopled with figures of national heroes, into the country's foremost place politique", as historian Rodney Mace has written. Its symbolic importance was demonstrated in 1940 when the NaziSS developed secret plans to transfer Nelson's Column to Berlin following an expected German invasion, as related by Norman Longmate in If Britain Had Fallen (1972).

A major redevelopment of the square led by W.S. Atkins with Foster and Partners as sub-consultants was completed in 2003. The work involved closing the main eastbound road along the north side, diverting the traffic around the other three sides of the square, demolishing the central section of the northern retaining wall and inserting a wide set of steps leading up to a pedestrianised terrace in front of the National Gallery. The construction includes two lifts for disabled access, public toilets and a small café. Previously, access between the square and the gallery was by two crossings at the northeast and northwest corners of the square.[17]

Since 1998 the empty plinth in the north-west corner of the square – which has become known as the "Fourth Plinth" – has been used to show a series of specially commissioned artworks. The scheme was initiated by the Royal Society of Arts and continued by a Fourth Plinth Commission, appointed by the Mayor of London. A 1:30 scale replica of HMS Victory in a giant glass bottle by Yinka Shonibare was installed on the plinth in May 2010.[20][21]

A new sculpture on the fourth plinth was unveiled on 6 March 2015 called "The Gift Horse." The sculpture is a model of a horse's skeleton with a live display of the London Stock Exchange.[22]

On the south side of the square, on the site of the original Charing Cross, is a bronze equestrian statue of Charles I by Hubert Le Sueur. It was cast in 1633, and placed in its present position in 1678.[26]

In 1841, following suggestions from the local paving board, Barry agreed to the idea that two fountains should be installed, to counteract the effects of the reflected heat and glare from the proposed asphalt surface. The First Commissioner of Woods and Forests also welcomed the plan, not least because the fountains would reduce the open space available for public gatherings and thus reduce the risk of riotous assembly.[32] The fountains were fed from two wells, one in front of the National Gallery and one behind it, connected by a tunnel. The water was pumped by a steam engine, housed in a building behind the gallery.[6]

In the late 1930s it was decided to replace the pump and the centrepieces of the fountains. The new centrepieces, designed by Sir Edwin Lutyens, were intended as memorials to Lord Jellicoe and Lord Beatty, although the busts of the two admirals, initially planned to be placed in the surrounds of the fountains were eventually placed against the northern retaining wall when the project was completed after the Second World War. [33] The new fountains cost almost £50,000. The old ones were bought for presentation to the Canadian government, and are now in Ottawa and Regina.[34][35]

A further programme of restoration work was completed by May 2009. The existing pump system was replaced with a one capable of sending an 80-foot (24 m) jet of water into the air.[36] A new LED lighting system was also installed to reduce the cost of lighting maintenance. The new lighting was designed with the London 2012 Summer Olympics in mind, and for the first time could project many different combinations of colours on to the fountains.[34] The new lighting system has a much lower energy requirement and should reduce its carbon footprint by around 90%.[36]

The square was once famous for its feral pigeons, and feeding them was a popular activity. The desirability of the birds' presence was contentious: their droppings disfigured stonework, and the flock, estimated at its peak to be 35,000, was considered a health hazard. In February 2001, the sale of bird seed in the square was stopped[37] and other measures introduced to discourage the pigeons, including the use of trained birds of prey.[38] Groups of supporters continued to feed the birds, but in 2003 the then-Mayor, Ken Livingstone, enacted bylaws to ban the feeding of pigeons in the square.[39] In September 2007 Westminster City Council passed further bylaws banning the feeding of birds on the square's pedestrianised North Terrace and other pavements in the area.[40]

For many years, revellers celebrating the start of a New Year have gathered in the square, despite a lack of civic celebrations being arranged. The lack of official events in the square was partly because the authorities were concerned that actively encouraging more partygoers would cause overcrowding. Since 2003, a firework display centred on London Eye and the South Bank of the Thames has been provided as an alternative.

Victory in Europe Day (VE Day) was 8 May 1945, the date when the Allies during the Second World War celebrated the defeat of Nazi Germany. Trafalgar Square was filled with a crowd wanting to hear the formal announcement by Winston Churchill that the war was over. The square was also used as a place of celebration by people travelling there from all over the country. On 8 May 2005 the BBC held a concert to celebrate the 60th anniversary of VE Day.

There has been a Christmas ceremony at Trafalgar Square every year since 1947. A Norway spruce (or sometimes a fir) is given by Norway's capital Oslo and presented as London's Christmas tree, as a token of gratitude for Britain's support during World War II. (Besides the general war support, Norway's Prince Olav, as well as the country's government, lived in exile in London throughout the war.) As part of the tradition, the Lord Mayor of Westminster visits Oslo in the late autumn to take part in the felling of the tree, and the Mayor of Oslo then comes to London to light the tree at the Christmas ceremony.[41]

Switching on the lights of the giant Christmas tree has been part of Christmas season in London since 1947.[42] The ceremony is usually being held 12 days before 25 December. The festivity is open to the public and attracts a large number of people.[43] It is usually accompanied with several nights of Christmas carol singing programme, series of performances and events.[44] 2012 was the 65th time that London was presented with a Christmas tree from Oslo.[45] Performances during the ceremony included Norwegian trumpeter Tine Thing Helseth alongside the St. Martin-in-the-Fields choir and the Regend Hall Band of the Salvation Army.[46] According to the Norwegian foresters, only the best tree is presented from their forest for the people of London. Fondly described by the woodsmen who care for it as 'the queen of the forest', it can reach up to 80 feet in height and is between 50 and 100 years old. The giant Christmas tree is taken down on the 12th night of Christmas for recycling.[47]

Protesting against harassment of photographers under anti-terrorism law, 23 January 2010

Since its construction, Trafalgar Square has been a venue for political demonstrations, though the authorities have often attempted to ban them. The 1839 fountains were added on their current scale to reduce the possibility of crowds gathering in the square as they were not in the original plans.

By March of the year Nelson's Column opened, the authorities had started banning Chartist meetings in the square. A general ban on political rallies remained in effect until the 1880s, when the emerging Labour movement, particularly the Social Democratic Federation, began holding protests there.

On "Black Monday" (8 February 1886), protesters rallied against unemployment; this led to a riot in Pall Mall. A larger riot (called "Bloody Sunday") occurred in the square on 13 November 1887.

One of the first significant demonstrations of the modern era was held in the square on 19 September 1961 by the Committee of 100, which included the philosopher Bertrand Russell. The protesters rallied for peace and against war and nuclear weapons. In March 1968, a crowd of some 10,000 demonstrated at the square against US involvement in the Vietnam War, before marching to the American Embassy in Grosvenor Square.[48]

In December 2009, participants from the Camp for Climate Action occupied the square for the two weeks in which the UN Conference on Climate Change took place in Copenhagen.[51] It was billed as a UK base for direct action on climate change during the conference, and saw various actions and protests stem from the occupation.[52][53][54]

On 26 March 2011, the square was occupied by protesters using the square to protest against the UK Budget and its proposed budget cuts. During the night however, the situation turned violent as the escalation by riot police and protesters damaged portions of the square.[55]

On 6 July 2005 Trafalgar Square was a gathering place for the announcement on London's bid to host the 2012 Summer Olympics, Located also on Trafalgar Square is its Official Countdown Clock unveiled on 14 March 2011.

In May 2007, the square was grassed over with 2,000 square metres of turf for two days as part of a campaign by London authorities to promote "green spaces" in the city.[58]

Every year on the anniversary of the Battle of Trafalgar (21 October), the Sea Cadet Corps holds a parade in honour of Admiral Lord Nelson and the British victory over the combined fleets of Spain and France at Trafalgar. The Areas of the Sea Cadet Corps are represented by seven 24-cadet platoons. The National Sea Cadet Band also parades, as does a guard and colour party.

On 7 July 2011, due to building works in Leicester Square, the world premiere of the final film in the Harry Potter series, Harry Potter and the Deathly Hallows - Part 2, was held in Trafalgar Square, with a ¾ mile red-carpet linking the two squares. Fans camped in Trafalgar Square for up to three days before the premiere. It was the first premiere ever to be held in Trafalgar Square.

Charing Cross – Northern and Bakerloo Lines—has an exit in the square. The two lines originally had separate stations, of which the Bakerloo line one was called Trafalgar Square; they were linked and renamed in 1979 as part of the construction of the Jubilee line, which was later rerouted to Westminster tube station in late 1999.

There is a life scale replica of the square in Bahria Town, Lahore, Pakistan. The square is a well known tourist attraction in Bahria town Lahore and serves as a centre for the residents of nearby community.[63][64]

Stone to Build London: Portland's Legacy, Gill Hackman, Folly Books, Monkton Farleigh, 2014, ISBN978-0-9564405-9-4. Book includes details of the Portland stone buildings around Trafalgar Square, including St Martin in the Fields, the National Gallery and Admiralty Arch.